1. Field of the Invention
This invention relates generally to power control in a wireless communication device transmitter, and, more particularly, to fast, closed-loop power control for nonconstant envelope modulation formats.
2. Related Art
With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld telephone like communication handset. The different modulation and transmission schemes each have advantages and disadvantages.
In a typical global system for mobile communications (GSM) mobile communication system, a gaussian minimum shift keying (“GMSK”) modulation scheme supplies a low-noise phase modulated (“PM”) transmit signal to a non-linear power amplifier (PA) directly from an oscillator. In such an arrangement, a non-linear power amplifier, which is highly efficient, can be used thereby allowing efficient transmission of the phase modulated signal and minimizing power consumption. Because the modulated signal is supplied directly from an oscillator, the need for filtering, either before or after the power amplifier, is minimized.
Other transmission standards, however, require that both a PM signal and an amplitude modulated (“AM”) signal be transmitted. The presence of the AM component results in what is referred to as a “non-constant envelope” transmit signal. In other words, the amplitude of the transmit signal has an amplitude that varies over time. Standards such as these increase the data rate without increasing the bandwidth of the transmitted signal. An example of such a standard is IS-95, which employs a code division multiple access (CDMA) modulation methodology. Further, new standards are evolving that will also use both phase modulation and amplitude modulation. For example, the enhanced data rates for GSM evolution (EDGE) standard, which is an extension to the GSM standard, and standards employing wide band code division multiple access (WCDMA), use a non-constant envelope modulation scheme.
Unfortunately, the existing GSM modulation scheme is not easily adapted to transmit a signal that includes both a PM component and an AM component because the PA cannot be driven directly from an oscillator, as is used in GSM.
This condition is further complicated because transmitters typically employed in GSM and EDGE communication systems transmit in bursts and must be able to precisely control the fast ramp-up and ramp-down of the transmit power as well as have a high degree of control over the output power level over a wide power range. This power control is typically performed using a closed feedback loop in which a portion of the signal output from the power amplifier is compared with a reference signal and the resulting error signal is integrated and fed back to the control input of the power amplifier.
CDMA is an example of a modulation scheme including both a PM component and an AM component. CDMA systems also require that the transmit power be accurately controlled. However, CDMA systems generally do not employ burst-type transmission, and therefore have no requirement for fast ramp-up and ramp-down of the power, as is used in GSM and EDGE. In non-burst transmission systems, such as CDMA, the output power may be controlled by a feedback loop having a time-constant that is very low compared to the time-constant of the amplitude variations of the modulator. Therefore the power control occurs at a sufficiently slow rate that it does not strip the AM signal component of the CDMA signal.
Unfortunately, the transmit signal used in EDGE requires much faster power control for ramping power up and down, where the allowed ramp time is on the order of a few symbols. Therefore, EDGE cannot use the same slow power control scheme as is used in CDMA. Such power control would be too slow for the EDGE power ramp-up and ramp-down. Further, the speed of the power control cannot simply be increased, since when attempting to include a PM component and an AM component in a nonconstant envelope modulation format, the closed power control loop will tend to cancel the amplitude variations present in the transmit signal while attempting to maintain the desired output power.
In such transmission signals containing both PM and AM components, the output power can be controlled by applying a predetermined control voltage to the power amplifier. Unfortunately, this does not provide real-time power control. If the gain characteristic of the PA drifts over time or over temperature, no correction will be applied. This results in incorrect levels of power being transmitted.
Another known method to control the output power is to “predistort” the modulated signal in such a way that the power control loop will cancel the effect of the predistortion. In such a method, the amplitude information is passed through a transfer function that is the inverse of the power control loop transfer function. Unfortunately, this method is costly and inefficient.
Therefore, there is a need in the industry for a transmission technique in which a fast closed loop power control system is applied to a linear power amplifier in which a signal having both a PM component and an AM component is amplified.